In a gas transmission and distribution system, gas (i.e. fuel gas) is normally transmitted at high pressure, but has to be distributed to end users through local networks at a pressure which is relatively low and is also controlled fairly accurately. A pressure reducing station is therefore required for coupling the high-pressure transmission line to the local distribution network.
Conventionally, a pressure reducing station comprises two streams in parallel, each stream containing an initial isolating valve, slam-shut valve, a suitable number of pressure regulating (reducing) valves depending on the amount of pressure reduction required, and a final isolating valve. A typical example of such a station is shown in our GB 2 217 877 A. Such a station is normally located above ground, and the fact that the various valves in each stream are arranged in a linear sequence means that it normally occupies a good deal of space.
There has recently been increasing interest in the possibility of pressure reducing stations which are largely buried. A buried station has various advantages: it is relatively unobtrusive, the liability of accidental or deliberate damage to it is reduced, and the noise emission from it is reduced.
A buried station naturally requires a hole or pit to be dug, and it is desirable to minimize the size of this pit. A somewhat different design has therefore been developed for buried pressure reducing stations. The basic feature of a buried pressure reducing station, in its current form, is a pair of pressure chambers, an inlet chamber and an outlet chamber. The inlet chamber usually includes a filter, end communicates with the outlet chamber through a valve stack (a valve assembly or cartridge) including a slam-shut valve and a suitable number of pressure regulating valves.
Various specific designs for such stations are known. The two chambers may be physically separate units located adjacent to each other, or they may be separate units but located in a common outer enclosure. More usually, however, they are separate regions of a single pressure body divided by internal walls. It is also common for such stations to be largely buried, as discussed above.
The valve stack has controls which have to be accessible from outside the chamber. In addition, the valve stack requires servicing at suitable intervals. The design of the station should therefore be such as to make the valve stack easily accessible. This is commonly achieved by locating the chamber containing the valve stack towards the top of the pressure vessel and providing it with a lid which can be removed to provide access to the valve stack. This chamber therefore typically has a port or opening in its base with which the valve stack fits. The chamber containing the valve stack may be either the inlet or the outlet chamber.
The valve stack obviously has to have a good seal to the port between the two chambers, to prevent the escape of high-pressure gas through that port directly to the outlet.
It would be possible to bolt the valve stack directly to the chamber wall surrounding the port. However, this would require access to the bottom of the chamber to release the valve stack, and this would generally be difficult. Some other sealing technique is therefore required.
A technique for achieving such a seal is to form the port and the mating end of the valve stack as accurately formed cylindrical surfaces, with one of them including a suitable seal (such as an O ring). The valve stack is attached to the lid of the chamber, and after servicing, the combined lid and valve stack is lowered into the chamber, with the bottom end of the valve stack being inserted into the port. The lid can then be locked in position, so locating the valve stack in position. An example of this technique, which can be described as a piston-type seal, is the system described in GB 2 024 650 A.
This technique has various drawbacks. The seal between the valve stack and the port is expensive and difficult to maintain, and it is difficult to monitor the insertion of the valve stack into the port and avoid damage to the sealing parts of the valve stack and/or the port. Further, such a piston-type seal requires a substantial clearance between the two parts being sealed together, to accommodate the O sealing ring; hence if any damage (eg mechanical or chemical) occurs to the sealing ring causing it to disintegrate, a leakage path of substantial size will result.
The general object of the present invention is to provide an improved technique for sealing a valve stack to a port, particularly a port in a pressure reducing station.